The present invention relates to transfer mechanisms for feeding individual blanks of flexible sheet material from a blank pickup station to a blank discharge station. In particular, the invention is directed to a system including an inverting arm of effective constant pivotal length having a pivot location that varies as the arm transfers individual blanks between the pickup and the discharge locations. The invention will be specifically disclosed in connection with an inverting arm in an apparatus for pivotally moving an individual blank approximately 90.degree. from a storage magazine for a stack of individual blanks to a forming station where the blank is folded into a box or a carton structure.
It is well known in the art to stack a plurality of flat foldable paperboard blanks in a blank storage magazine of a box forming machine and to thereafter separate and remove the forward blank from the stack for transfer to a forming station. Most commonly, the blank is engaged by suction cups on a feed carrier during removal from the stack and transfer movement. As the feed carrier is moved to the forming die, the blank is inverted and then placed on the forming die, vacuum in the suction cups is then terminated and the blank is released from the feed carrier.
One example of a blank forming machine is disclosed in U.S. Pat. No. 3,269,724 wherein the feed carrier engaging the blank is secured to a carriage. The carriage has rollers which are movingly disposed in cam tracks to guide the carriage and attached feed carrier upwardly and downwardly between a magazine supporting a stack of paperboard blanks and a forming die. A plunger cooperatively interacts with the die and folds the flat paperboard blanks into a box like or carton structure.
The most prevalent design in contemporary carton forming machines has a driven feed carrier mounted on the end of a pivoting inverting arm. In this design, the feed carrier also has suction cups for engaging and removing the individual blanks from the stack of blanks in a magazine holder. In this last mentioned design, the inverting arm is used to pivotally move the feed carrier and vacuum secured blank through approximately 90.degree. from the magazine to the forming station. However, inasmuch as the blanks must first be removed from the magazine and thereafter positioned in a forming station, the transfer movement of the blank is not a simple arc. Instead, it is desirable to extend and retract the inverting arm along a relatively straight line at both the magazine pickup location and the forming means discharge location.
In order to effectuate the combined pivoting and extension/retraction movements, the inverting arms of the prior art have guideways slideable on fixed studs. These guideways allow longitudinal sliding movement of the inverting arms relative to the studs. However, during the feed movement of the inverting arm, in this prior design, the arm is longitudinally moved with respect to the pivoting stud. Simultaneously, the inverting arm is pivoted about this same stud. Hence, the pivot point of the inverting arm is fixed, but the arm's effective pivotal length varies. Unfortunately, this variation in the inverting arm's length adds unwanted speed and acceleration to the feed movement of the paperboard blank during the middle portion of the cycle. Examples of this prevalent fixed pivot, but variable pivot length design are disclosed in U.S. Pat. Nos. 2,624,249; 2,805,060 and 3,176,978.
A further type of inverting arm design is found in U.S. Pat. No. 3,008,385. In this design, an inverting arm has a track guide disposed about a pivotal roller for guiding both longitudinal movement of the arm relative to the pivotal roller and pivotal movement. Unlike the fixed pivot designs discussed above, however, this pivotal roller is mounted about a bell crank and is not fixed relative to the machine base. Nevertheless, the inverting arm moves longitudinally with respect to the pivotal roller and the pivotal length of the inverting arm is variable throughout the feed movement.